Method of producing low-line aluminous cement from aluminum smelting residue

ABSTRACT

A certain amount of a starting calcarious material is added to an aluminum smelting residue, the mixture is smelted or semi-smelted, air is blown to the smelted or semi-smelted product to oxidize A/N, and the smelted or semi-smelted product is finely pulverized to obtain a low-lime aluminous cement. 
     An excellent refractory aluminous cement can be prepared according to this method.

BACKGROUND OF THE INVENTION

The present invention relates to a method of producing a low-lime aluminous cement from an aluminum smelting residue. More particularly, the present invention relates to an improvement in the method disclosed by us in U.S. Pat. No. 4,071,373.

Aluminum smelting residues produced as by-products in the aluminum industry are partially used for the preparation of heat-insulating materials and slag-forming agents to be used in the industry of iron and steel. However, the amounts used of these aluminum smelting residues have recently been decreasing. Furthermore, since these residues contain aluminum nitride and aluminum carbide, they generate ammonia gas and unsaturated hydrocarbons and emit bad smells on contact with rain water or sea water. Accordingly, from the viewpoint of prevention of environmental pollution, it is not permissible to discard them simply as industrial wastes.

SUMMARY OF THE INVENTION

We made researches with a view to developing a method of producing a low-lime aluminous cement having a high refractoriness and a long-durable strength by utilizing aluminum smelting residues, the disposal of which involves the above-mentioned problems, as a valuable Al₂ O₃ resource replacing bauxite without causing environmental pollution. As the result, we have now completed the present invention.

More specifically, in accordance with the present invention, there is provided a method of producing a low-lime aluminous cement from an aluminum smelting residue, which comprises mixing an aluminum smelting residue with a starting calcareous material and smelting or semi-smelting the mixture, wherein the starting calcareous material is added in such an amount, relatively to SiO₂, Al₂ O₃ and MgO contained in the aluminum smelting residue, that the smelted or semi-smelted mixture of the aluminum smelting residue and starting calcareous material has a composition of 2CaO.Al₂ O₃.SiO₂, MgO.Al₂ O₃ and CaO.nAl₂ O₃ and the CaO content in said mixture is 20 to 30% by weight, the smelted or semi-smelted product formed by smelting or semi-smelting the mixture is pulverized by strongly blowing air to the smelted or semi-smelted mixture to oxidize AlN, and the pulverized product is then finely divided. The mixture of the aluminum smelting residue and the starting calcarious material may be smelted or semi-smelted at a temperature of 1400° to 2100° C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An aluminum smelting residue is as ash rising to the surface of a melt when an aluminum scrap or alloy is remolten. Ordinarily, this ash is scraped out from the surface of the molten metal, is then squeezed once or twice to recover the residual metal and is pulverized. This aluminum smelting residue contains various metal oxides as impurities, and though the chemical composition differs to some extent according to the kind of the starting metal material to be smelted, the aluminum smelting residue has the following main ingredients.

Chemical Composition of Aluminum Smelting Residue

    ______________________________________                                                SiO.sub.2   less than 10%                                                      Al.sub.2 O.sub.3                                                                           65-95%                                                             Fe.sub.2 O.sub.3                                                                           less than 5%                                                       MgO         less than 20%                                                      Al          less than 20%                                                      AlN         less than 20%                                                      Others      less than 5%                                                ______________________________________                                    

In case of a starting metal material having a high Si content, Si is intruded in the form of SiO₂ into the residue, and in case of a starting metal material having a high Mg content, Mg is included in the residue in the form of free MgO or spinel. Since residual Al is not completely recovered by squeezing, Al is included in the residue in the form of aluminum powder, and AlN is formed by reaction of Al with nitrogen in air while the starting metal material is smelted.

When an aluminous cement is prepared from such aluminum smelting residue and a starting calcareous material, since SiO₂, MgO and other metal oxides are contained in much larger quantities in the starting material than in a bauxite material, if CaO is included in a large amount, 2CaO.SiO₂, 12CaO.7Al₂ O₃ or free MgO are included in a clinker and other metals are made free in the clinker, with the result that the quality of a cement is degraded. Accordingly, a special care should be taken for eliminating bad influences of SiO and MgO contained in the starting material. Therefore, the method of the present invention is directed to the production of a special aluminous cement having a composition different from that of an ordinary aluminous cement. Incidentarily, the chemical composition of an ordinary aluminous cement is usually as follows:

Chemical Composition of Ordinary Aluminous Cement

    ______________________________________                                                SiO.sub.2   less than 8%                                                       Al.sub.2 O.sub.3                                                                           40-65%                                                             Fe.sub.2 O.sub.3                                                                           less than 15%                                                      CaO         35-45%                                                             MgO         less than 5%                                                ______________________________________                                    

The ingredient compounds of this ordinary aluminous cement are CaO.Al₂ O₃, 12CaO.7Al₂ O₃, 2CaO.SiO₂, 4CaO.Al₂ O₃. Fe₂ O₃ and free MgO. In the present invention, 12CaO.7Al₂ O₃, 2CaO.SiO₂ and free MgO in the above composition are eliminated and CaO.Al₂ O₃ or CaO.2Al₂ O₃, 2CaO.Al₂ O₃.SiO₂ and MgO.Al₂ O₃ are included instead.

2CaO.SiO₂ is a main hydraulic compound of Portland cement. When this compound is included in an aluminous cement, Ca(OH)₂ formed by the hydration action of this compound makes the hydration action of CaO.Al₂ O₃ abnormal to render the cement quick setting and quick hardening and reduce the strength. Accordingly, it is preferred that the content of this compound be small.

In the present invention, SiO₂ is converted to gehlenite, no particular disadvantage is brought about even if the content of SiO₂ in the starting material is relatively high. It is known that 2CaO.Al₂ O₃.SiO₂ has a latent hydraulic property. This compound does not exhibit the hydration action directly but exerts the hydration action over a long period. Accordingly, this compound is effective for manifesting the long-period strength in an aluminous cement. The long-period strength of an aluminous cement ordinarily tends to decrease. Therefore, incorporation of 2CaO.Al₂ O₃.SiO₂ in an aluminous cement results in improvements of properties of the aluminous cement and bad influences of 2CaO.SiO₂ are eliminated by incorporation of this compound.

In 2CaO.SiO₂, the contents of CaO and SiO₂ are 65.1% by weight and 34.9% by weight, respectively. In CaO.Al₂ O₃, the contents of CaO and Al₂ O₃ are 35.4% by weight and 64.6% by weight, respectively. In CaO.2Al₂ O₃, the contents of CaO and Al₂ O₃ are 21.5% by weight and 78.5% by weight, respectively. Furthermore, in 2CaO.Al₂ O₃.SiO₂, the contents of CaO, Al₂ O₃ and SiO₂ are 40.9% by weight, 37.2% by weight and 21.9% by weight, respectively, and in MgO.Al₂ O₃, the contents of MgO and Al₂ O₃ are 28.2% by weight and 71.8% by weight, respectively.

In the CaO content in the clinker is higher than 35% by weight, undesirable 2CaO.SiO₂ is formed in the aluminous cement, and 3CaO.Al₂ O₃ (52.2% by weight of CaO and 37.8% by weight of Al₂ O₃) or 12CaO.7Al₂ O₃ (48.5% by weight of CaO and 51.5% by weight of Al₂ O₃) having highly quick setting and hardening properties is formed. Furthermore, MgO is rendered free to degrade the properties of the aluminous cement. Moreover, if MgO.Al₂ O₃ or 2CaO.Al₂ O₃.SiO₂ is formed in the aluminous cement, since Al₂ O₃ in the clinker is consumed, the ratio of CaO to Al₂ O₃ is increased above 1.

In view of the foregoing facts, in order to include CaO.Al₂ O₃, 2CaO.Al₂ O₃.SiO₂ and MgO.Al₂ O₃ in an aluminous cement produced from an aluminum smelting residue, it is preferred that the CaO content in the clinker be lower than 35% by weight, and in order to include CaO.2Al₂ O₃ in the aluminous cement, it is preferred that the CaO content be smaller than 30% by weight. Accordingly, if the residue is composed solely of pure Al₂ O₃, the CaO content can be increased to 35% by weight. However, since SiO₂ or MgO is ordinarily included in the residue, it is indispensable that the CaO content in the clinker should be controlled to 20 to 30% by weight. If the CaO content is thus controlled, all of MgO is combined with Al₂ O₃ to form MgO.Al₂ O₃ and no free MgO is formed. The melting point of MgO.Al₂ O₃ in the range of from 2000° to 2100° C. and this compound imparts a fire resistance to the aluminous cement and has no bad influences on the water hardenability of the aluminous cement.

Metallic aluminum in the starting residue is oxidized to Al₂ O₃ during the step of sintering and smelting the clinker. Accordingly, such Al is valuable as the Al₂ O₃ source. Although AlN is partially oxidized, it is left in the clinker in a rertain amount, and it is necessary to remove this residual AlN.

If AlN is left in the clinker, when a cement obtained by pulverizing the clinker is hydrated, ammonia gas is formed by the following reaction to cause environmental pollution and the hardened cement is rendered coarse by formation of bubbles:

    AlN+3H.sub.2 O→NH.sub.3 +Al(OH).sub.3

We found that when a smelted or semi-smelted clinker is taken out from a furnace, if a large quantity of air is blown to the clinker in the red-hot state, the residual AlN is oxidized according to the following reactions and the residual AlN can be removed;

    2AlN+30.sub.2 =Al.sub.2 O.sub.3 +N.sub.2 O.sub.3

    2N.sub.2 O.sub.3 +O.sub.2 =4NO.sub.2

Namely, the residual AlN is oxidized by oxygen contained in air to Al₂ O₃ and NO₂ gas. Al₂ O₃ released by the above reactions is combined with lime at high temperatures during cooling to promote formation of CaO.Al₂ O₃ or CaO.2Al₂ O₃.

The method of producing an aluminous cement from an aluminum smelting residue according to the present invention will now be described in detail with reference to the following Examples.

EXAMPLE 1

The chemical composition of the aluminum smelting residue used is as follows:

    ______________________________________                                                ignition loss                                                                          0.8%                                                                   SiO.sub.2                                                                              4.7%                                                                   Al.sub.2 O.sub.3                                                                       81.3%                                                                  Fe.sub.2 O.sub.3                                                                       4.6%                                                                   MgO     10.9%                                                                  TiO.sub.2                                                                              1.2%                                                                   Metallic Al                                                                            11.1%                                                                  AlN     12.4%                                                           ______________________________________                                    

Based on the assumption that SiO₂ is combined with CaO and Al₂ O₃ to form 2CaO.Al₂ O₃.SiO₂ and a part of remaining Al₂ O₃ is combined with MgO to form MgO.Al₂ O₃, the amount of lime necessary for formation of CaO.Al₂ O₃ by reaction of the remainder of Al₂ O₃ with CaO is calculated. It is found that the amount of lime is 60.4 parts per 100 parts of the aluminum smelting residue. The chemical composition of a clinker obtained by sintering or smelting this composition is as follows:

Chemical Composition of Clinker

    ______________________________________                                                 SiO.sub.2                                                                            3.5%                                                                     Al.sub.2 O.sub.3                                                                     60.7%                                                                    Fe.sub.2 O.sub.3                                                                     3.4%                                                                     CaO   25.3%                                                                    MgO   8.1%                                                                     TiO.sub.2                                                                            0.9%                                                             ______________________________________                                    

When this clinker is subjected to the X-ray analysis, it is found that the clinker comprises 0.3% of AlN, 28.7% of MgO.Al₂ O₃, 15.9% of 2CaO.Al₂ O₃.SiO₂, 52.8% of CaO.Al₂ O₃ and a trace of CaO.2Al₂ O₃ with the total being at least 98.8%. This clinker is finely divided to have a Blaine value of about 4000 cm² /g. The results of the physical tests made on the so-obtained aluminous cement are as follows.

Setting Time:

Initial: 1 hour and 15 minutes

Final: 2 hours and 35 minutes

Compressive Strength (Kg/cm²):

1 day: 415

3 days: 429

7 days: 457

28 days: 524

When a cement mortar specimen is prepared by kneading the so obtained cement with water and sand, no ammonia smell is produced. Accordingly, it is confirmed that an excellent heat-resistance special aluminous cement producing no bad smell is obtained in this Example. Although the melting point of an ordinary aluminous cement is 1400° to 1600° C., the special aluminous cement of the present invention, which is different from the ordinary aluminous cement in the chemical composition and mineral composition, has a melting point of 1600° to 1800° C. and it exhibits a very high fire resistance.

EXAMPLE 2

The chemical composition of the aluminum smelting residue used is as follows:

    ______________________________________                                                Ignition loss  0.8%                                                            SiO.sub.2      4.7%                                                            Al.sub.2 O.sub.3                                                                              81.3%                                                           Fe.sub.2 O.sub.3                                                                              4.6%                                                            MgO            10.9%                                                           TiO.sub.2      1.2%                                                            Total          103.5%                                                   ______________________________________                                    

Quick lime is incorporated in this residue in an amount of 33.8 parts per 100 parts of the residue, and the mixture is charged in a Soderberg type electric furnace and smelted at a temperature higher than 1800° C. When the melt is discharged from the furnace, high pressure air is blown to the melt to convert the melt to small masses having a length of 1 to 3 cm and a width of 0.1 to 0.6 cm. The small masses are rapidly cooled to form a vitreous clinker. N in AlN is thus completely removed and Al₂ O₃ formed by oxidation is combined with CaO. The chemical composition of the so obtained clinker is as follows:

    ______________________________________                                                 SiO.sub.2    3.5%                                                              Al.sub.2 O.sub.3                                                                            60.7%                                                             Fe.sub.2 O.sub.3                                                                            3.4%                                                              CaO          25.3%                                                             MgO          8.1%                                                              TiO.sub.2    0.9%                                                      ______________________________________                                    

When the clinker is subjected to the X-ray analysis, it is found that the clinker has the following mineral composition.

    ______________________________________                                         MgO . Al.sub.2 O.sub.3, spinel                                                                          28.7%                                                 2CaO . Al.sub.2 O.sub.3 . SiO.sub.2, gehlenite                                                          15.9%                                                 CaO . Al.sub.2 O.sub.3, calcium aluminate                                                               52.9%                                                 Total                    98.8%                                                 ______________________________________                                    

The clinker is finely divided to have a Blaine value of about 4000 cm² /g, and the physical properties are determined to obtain the following results.

Setting Time:

Initial: 1 hour and 15 minutes

Final: 2 hours and 35 minutes

Compressive Strength (Kg/cm²):

1 day: 415

3 days: 429

7 days: 457

28 days: 489

When the resulting cement is mixed with water, formation of ammonia is not observed.

From the foregoing test results, it is confirmed that a high-quality aluminous cement is obtained in this Example.

EXAMPLE 3

Quick lime is incorporated into an aluminum smelting residue in an amount of 18.5 parts per 81.5 parts of the residue, and the mixture is charged in a Soderberg type electric furnace and smelted. Oxygen is blown into the melt and the melt is discharged from the furnace.

The chemical composition of the aluminum smelting residue used is as follows:

    ______________________________________                                                Ignition loss  1.9%                                                            SiO.sub.2      0.1%                                                            Al.sub.2 O.sub.3                                                                              80.0%                                                           MgO            14.9%                                                           Fe.sub.2 O.sub.3                                                                              0.8%                                                            TiO.sub.2      0.7%                                                            AlN            22.8%                                                           Total          98.4%                                                    ______________________________________                                    

Since the residue used contains a large amount of AlN, oxygen is blown into the melt in the furnace to oxidize AlN. Furthermore, air is blown to the discharged melt outside the furnace to rapidly cool the melt, whereby a sponge-like porous solid is obtained. The results of the chemical analysis of the resulting clinker is as follows:

    ______________________________________                                                 SiO.sub.2    trace                                                             Al.sub.2 O.sub.3                                                                            68.0%                                                             CaO          20.1%                                                             MgO          11.9%                                                             AlN           0%                                                       ______________________________________                                    

From the results of the X-ray analysis, it is confirmed that the main minerals of the clinker are CaO.Al₂ O₃ and CaO.2Al₂ O₃ and that also MgO.Al₂ O₃ is contained in a certain amount but 2CaO.Al₂ O₃.SiO₂ is not contained.

The clinker is finely divided to have a Blaine value of 4000 cm² /g and the hydraulic properties of the resulting cement are determined to obtain the following results.

Setting Time:

initial: 1 hour and 5 minutes

Final: 3 hours and 15 minutes

Compressive Strength (Kg/cm²):

1 day: 281

3 days: 296

7 days: 316

28 days: 320

When the refractoriness of the cement is examined by using a Seger cone, it is found that the refractoriness is No. 36 (1790° C.).

Thus, it is confirmed that a valuable refractory aluminous cement is produced from the aluminum smelting residue in this Example.

As will be apparent from the foregoing description, in the method of producing a low-lime aluminous cement according to the present invention, an excellent aluminous cement can be produced by using an aluminum smelting residue which is an industrial waste causing environmental pollution and is not utilized but discarded, and the resulting low-lime aluminous cement does not cause environmental pollution and is effectively used not only as an ordinary construction material but also as a refractory. Accordingly, the present invention makes great industrial contributions. 

What is claimed is:
 1. A method of producing a low-lime aluminous cement from an aluminum smelting residue, which comprises mixing an aluminum smelting residue with a starting calcareous material and smelting or semi-smelting the mixture, wherein the starting calcareous material is added in such an amount, relatively to SiO₂, Al₂ O₃ and MgO contained in the aluminum smelting residue, that the smelted or semi-smelted mixture of the aluminum smelting residue and starting calcareous material has a composition of 2CaO. Al₂ O₃.SiO₂, MgO.Al₂ O₃ and CaO.nAl₂ O₃ and the CaO content in said mixture is 20 to 30% by weight, the smelted or semi-smelted product formed by smelting or semi-smelting the mixture is pulverized by strongly blowing air to the smelted or semi-smelted mixture to oxidize AlN, and the pulverized product is then finely divided.
 2. A method of producing a low-lime aluminous cement from an aluminum smelting residue according to claim 1, wherein the aluminum smelting residue is mixed with the calcareous material at such a mixing ratio that the smelted or semi-smelted product has a composition of 2CaO.Al₂ O₃.SiO₂, MgO.Al₂ O₃ and CaO.Al₂ O₃.
 3. A method of producing a low-lime aluminous cement from an aluminum smelting residue according to claim 1, wherein the aluminum smelting residue is mixed with the calcareous material at such a mixing ratio that the smelted or semi-smelted product has a composition of 2CaO.Al₂ O₃.SiO₂, MgO.Al₂ O₃ and CaO.2Al₂ O₃.
 4. A method of producing a low-lime aluminous cement from an aluminum smelting residue according to any of claims 1 through 3, wherein the aluminum smelting residue has a chemical composition comprising 0.8% of ignition loss, 4.7% of SiO₂, 81.3% of Al₂ O₃, 4.6% of Fe₂ O₃, 10.9% of MgO, 1.2% of TiO₂, 11.1% of metallic Al and 12.4% of AlN.
 5. A method of producing a low-lime aluminous cement from an aluminum smelting residue according to any of claims 1 through 3, wherein the aluminum smelting residue has a chemical composition comprising 0.8% of ignition loss, 4.7% of SiO₂, 81.3% of Al₂ O₃, 4.6% of Fe₂ O₃, 10.9% of MgO and 1.2% of TiO₂.
 6. A method of producing a low-lime aluminous cement from an aluminum smelting residue according to any of claims 1 through 3, wherein the aluminum smelting residue has a chemical composition comprising 1.9% of ignition loss, 0.1% of SiO₂, 80.0% of Al₂ O₃, 14.9% of MgO, 0.8% of Fe₂ O₃, 0.7% of TiO₂ and 22.8% of AlN.
 7. A method of producing a low-lime aluminous cement from an aluminum smelting residue according to any of claims 1 through 3, wherein the mixture of the aluminum smelting residue and the starting calcarious material is smelted or semi-smelted at a temperature of 1400° to 2100° C.
 8. A method of producing a low-lime aluminous cement from an aluminum smelting residue according to any of claims 1 through 3, wherein high-pressure air is blown to the smelted or semi-smelted product of the mixture of the aluminum smelting residue and the starting calcarious material to convert the smelted or semi-smelted product into small masses having a length of 1 to 3 cm and a width of 0.1 to 0.6 cm under tapid cooling.
 9. A method of producing a low-lime aluminous cement from an aluminum smelting residue according to any of claims 1 through 3, wherein oxygen is blown to the smelted or semi-smelted product of the mixture of the aluminum smelting residue and the starting calcarious material. 